This invention relates generally to a termination for high impedance attenuator, and more specifically, to an oscilloscope input circuit including a switchable high impedance attenuator section and an input terminator of such attenuator section.
General purpose electronic test and measurement instruments such as, for example, oscilloscopes are normally designed to include as part of the input circuit thereof switchable attenuators to accept a wide range of input signal voltages. Such attenuators preferably have high input resistance such as 1M.OMEGA., thereby reducing the loading effect upon the signal source to be measured. Such input circuits exhibit high inherent capacitance (C) which, in combination with the high input resistance (R) of attenuators, forms an RC network which limits the bandwidth, thereby rendering the instrument unsuitable for measuring the terminated end of a wideband transmission line of typically 50.OMEGA. characteristic impedance (Z.sub.0).
One conventional approach to correct this problem is to utilize a switchable low impedance (e.g. 50.OMEGA.) terminator at the input side of such attenuators as shown in FIG. 1. An input signal applied to input connector 10 is attenuated to a proper amplitude by switchable 1M.OMEGA. attenuator 12 before being applied to buffer amplifier 14 including inherent input capcitance 18. The output from buffer amplifier 14 is further amplified by amplifiers connected to output terminal 16. Attenuator 12 may be a high frequency type fabricated on a ceramic substrate by thick film technique as disclosed in U.S. Pat. No. 3,753,170, assigned to the assignee of the present invention. A series combination of switch 22 and termination resistor 20 is connected between the input end of attenuator 12 and ground. Switch 22 is normally open to reduce the loading effect but is closed when measuring terminated ends or low impedance wideband signal sources.
Another conventional approach is shown in FIG. 2, which utilizes a switchable low impedance input attenuator section 24 and high impedance input attenuator section 26 comprising high impedance attenuator 25 and impedance converter 27. The input signal applied to input terminal 10 is transmitted to low impedance common attenuator section 30 either through input attenuator section 24 or 26 depending on selection switch 28 before reaching terminal 16 via buffer amplifier 14. Attenuator sections 23, 25 and 30 are typically ganged together to allow single knob control.
Disadvantages of these prior arts are large reflections and complicated mechanical/electrical construction. That is, attenuator 12 is fabricated along a transmission line of a uniform characteristic impedance. When a step pulse A as shown in FIG. 4 is applied to input terminal 10, such pulse is reflected to provide a negative reflection as represented by waveform B in FIG. 4 because of inherent capacitance 18. The time T represents the delay time of the transmission line of such attenuator 12. This causes an adverse effect on the signal source.